The invention relates to a process for placing a bundle of rods arranged in the form of a square pitch spacing lattice in a nuclear fuel assembly in a case. The invention also also relates to an installation for performing this process.
In nuclear reactors, particularly of the pressurized water type, the fissile material is constituted by fritted uranium dioxide (U0.sub.2) pellets, which are stacked in metal cans for forming fuel rods. The fuel rods are themselves arranged in bundles in accordance with a square spacing lattice in nuclear fuel assemblies, which are then juxtaposed for forming the reactor core.
FIG. 1 is a perspective view diagrammatically showing a nuclear fuel assembly. Apart from the bundle of rods a, it comprises a certain number of guide tubes b occupying the space of certain rods a in the square pitch or spacing lattice. In order to clarify the drawing, only some rods and some guide tubes are shown. Most of the guide tubes are used for slidingly guiding the absorbing rods of the not shown control groups.
The relative positioning between the rods and the tubes is ensured by spacing grids c, of which there are generally eight and whereof only three are shown in FIG. 1. These grids constitute the framework of the assembly.
The assembly also comprises two end pieces constituted by an upper fitting d and a lower fitting e, said fittings being respectively used for the handling of the assembly and the fixing thereof in the reactor core. They are fixed to the ends of the guide tubes b.
When such an assembly has been irradiated in the nuclear reactor core, it has to be dismantled to enable the fuel to be removed.
A first stage in the dismantling of assemblies consists of cutting up the guide tube b at a level below that of the upper end of the rods a, so as to give access to said rod end, following the removal of the upper fitting d from the upper grid and the parts of the guide tubes attached thereto. During a second stage, rods a are grasped by their ends, so as to be extracted from the remainder of the framework of the assembly.
When the rods have been extracted, the actual dismantling is completed and the rods are generally stored in a pool to enable their activity to decay. For this purpose they are placed in cases. The invention specifically relates to a process and installation making it possible to place the bundle of rods in a case following the extraction thereof.
When the decay in the activity of the nuclear fuel contained in the rods is considered to be adequate, the latter are cut and the fuel contained therein is dissolved prior to removal.
The function of placing the rods in the case prior to the storage thereof serves to reduce to the maximum the overall dimensions of the rods stored, whilst making it easily possible to manage said rods. To this end, all information concerning the life of the rods (irradiation time in the reactor, date of placing in the case, etc) generally appear on the case. The operation of placing the rods in the case must consequently make it possible to occupy in an optimum manner the internal volume of the cases.
In the present state of the art, said operation is performed by introducing each of rods vertically into an individual guide, whose diameter decreases towards the bottom, the lower ends of the guides also approaching one another towards to the bottom, above the vertically positioned case to be filled.
However, installations of this type suffer from the disadvantage of making it necessary to individually push on each of these rods, which is very difficult to carry out, in view of the deformations of some of these after irradiation. Moreover, the passage of each of the rods into an individual guide having at its lower end a diameter which is scarcely larger than the nominal diameter of the rods leads to a risk of the latter jamming, due to their swelling under irradiation.
In EP-A-0 06695, the rods are moved together by means of a complex device, within which the rods are vertically pushed. This device is in three parts successively ensuring the shape change of the bundle, the moving together of the rods contained in each row and then the moving together of the rows of rods. However, the first part of this device has individual guide tubes into which the rods pass, so that the aforementioned disadvantages still exist.
The present invention relates to a process and installation for placing in a case a bundle of rods, whilst eliminating any risk of these rods jamming and whilst still ensuring a completely effective filling of the cases.